1. Field
This disclosure relates generally to lasers and more particularly to combining multiple laser oscillators to produce a higher intensity output.
2. Description of Related Art
Many potential laser applications such as laser communications, industrial material processing, and remote sensing require the use of laser sources producing high brightness light. High brightness can be generally defined as high power per unit area per unit bandwidth per unit solid angle. Producing high brightness from a single laser source is generally limited by the fact that there is an inherent limit to the power or energy per unit volume that can be extracted from or stored in a lasing medium. Alternatively, high brightness laser output may be obtained from an array of coherent lasers that have approximately the same phase.
One method known in the art for providing a higher power laser output comprises directing the output from a master laser oscillator to several laser gain elements. U.S. Pat. No. 4,757,268, issued Jul. 12, 1988 to Abrams et al., describes such a laser apparatus with N parallel laser gain elements. If the outputs of the N laser gain elements sum incoherently, a brightness equal to N times the brightness of a single laser gain element results. However, in Abrams et al., phase conjugate reflector means are disposed in the optical path of the laser gain elements to provide that the laser beams traveling through the individual laser gain elements sum coherently. This coherent summation of the laser beams provide that the resultant laser apparatus output will have a peak brightness proportional to N2 times the brightness of a single laser gain element. Hence, coherent combination of laser outputs provides for substantial increases in laser output brightness.
The laser apparatus described by Abrams et al. uses a master oscillator comprising laser devices such as Nd:YAG crystals or diode lasers and several additional optical elements to ensure that the light traveling within the apparatus is properly polarized and directed. Hence, the apparatus described by Abrams may be expensive and difficult to implement.
High power laser systems utilizing a fiber laser as a master oscillator are also known in the art. Fiber lasers are relatively compact and efficient, which reduces the power and weight requirements for systems based on fiber lasers. However, the power output of a single fiber laser without amplification or other power increasing techniques is relatively low. U.S. Pat. No. 6,366,356, issued Apr. 2, 2001 to Brosnan et al., discloses a laser system using a diode pumped fiber laser as a master oscillator and a plurality of fiber amplifiers connected to the master oscillator. The outputs from the plurality of fiber amplifiers are collimated by a lens array to produce a single high power laser beam output.
As briefly described above, coherent combination of multiple laser beams provides a power-law increase in power output. Therefore, Brosnan describes an additional electronic apparatus to correct the phase of the output provided by each fiber amplifier. The ability to compensate for the relative optical phase shifts among the array of fiber amplifiers provides for the preferred coherent combination of outputs. However, the additional circuitry required to detect and compensate for the relative optical phase shifts increases the complexity of the system disclosed by Brosnan. Also, fiber amplifiers are generally less efficient than fiber oscillators (lasers). Therefore, the array of fiber amplifiers disclosed by Brosnan would provide less power than an array of fiber oscillators of the same number. Hence, the system disclosed by Brosnan would be considered less efficient than a system based on a plurality of fiber oscillators.
Other high power laser systems based on fiber lasers avoid fiber amplifiers by using multiple-core coupler fiber oscillators. U.S. Pat. No. 5,566,196, issued Oct. 15, 1996 to Scifres, describes a fiber laser with two or more generally parallel, nonconcentric doped core regions. The use of multiple cores spreads the light over a larger area of the fiber, thereby reducing the laser power density and reducing the nonlinear optical effects that would otherwise occur at high light intensities. Scifres discloses that the cores may be positioned far enough apart to ensure that light propagating in one core intersects only minimally with light propagating in the other cores, so that each core forms a completely independent laser. However, this configuration does not provide for phase-locking between the light propagating in each of the cores. Scifres also discloses spacing the neighboring cores sufficiently close such that interaction of the light in the cores does occur, thereby providing a phase-locked array of laser emitters in the fiber.
A key problem with multiple-core fiber oscillator systems, such as the system disclosed by Scifres, is heat dissipation. Since the cores are disposed parallel and adjacent to each other along the entire active region of the cores, the heat from each core will be partially transmitted to the adjacent cores. Hence, the power of the multiple-core fiber oscillator systems will be limited by the ability to dissipate the heat generated by the active regions away from the multiple-core fiber, similar to the way that glass rod lasers are limited in average power scaling.
U.S. Pat. No. 6,272,155, issued Aug. 7, 2001, to Sekiguchi describes the creation of a high intensity optical source through the creation of a high density group of incoherent fibers. See, for example, FIG. 3 of U.S. Pat. No. 6,272,155. If the fibers do not interact, they will lase with their own characteristic frequencies (spectrum of longitudinal modes) and thereby be incoherent. Sekiguchi discloses that the fibers are to be positioned relative to one another such that they do not interact. The total power output will then increase proportional to the number of sources (N) simply due to energy conservation.
Therefore, there exists a need in the art for a laser system that incorporates the use of low cost and efficient fiber lasers to generate a higher power laser output or outputs, while reducing the limitations on power output caused by heat dissipation and avoiding the complexity of electronic compensation of the outputs of the fiber lasers.